Perth was the kick-off for the Australian leg of the DISC. Doug Oldenburg and I left summer behind in Vancouver and flew to Brisbane and then hopped (5 hrs!) across Australia to Perth. Perth is a mining centre in Australia, home to several large mining companies including Rio Tinto, BHP Billiton, and Woodside among others. There were a couple participants from the major mining companies, but the main contingent included geoscientists involved in consulting for the mining industry. There are also several major universities, research institutes and government organizations in the area, including the University of Western Australia, Curtin University, and CSIRO. Mining is an area of active research, there is some interest in oil and gas as well as geothermal, and as in many locations, groundwater is an increasingly important application of electromagnetic methods. Often airborne EM, either time or frequency, is a method of choice when possible as it requires less permits than when ground access is required. One of the major challenges in applying EM in Australia is the presence of the conductive, highly variable, regolith that blankets the continent. The presence of this layer makes it challenging to get EM energy beneath to “light up” potential targets and characterize the geologic setting.
Day 1: DISC Course
The course started at 8am and we began with a brief introduction from each of the 30 participants on what they are working on. Following the introduction to DISC 2017, we jumped in to Direct Current (DC) resistivity — you can read about it in our presentation highlight blog.
From the participants:
“Well organized, great illustrations, good range of case studies, minimal equations — great!”
“I enjoyed Doug’s presentations. A lot was covered, with plenty of real-world case studies. Excellent value!”
“The apps are fantastic → This will be extremely useful”
“Lots of the simulation examples were very useful to get an image of what’s happening.”
“Very insightful and inspirational course.”
“I am impressed with the many applications of this method as well as your learning materials. Your open source materials are great and will help a lot in learning EM methods”
“Very good refresher for me (returning after 10+ years to research). Pleasing to meet & interact with the next generation of EM / Geophysics scientists who will map / explore Australia”
All of the course material is at: http://disc2017.geosci.xyz/perth.
Day 2: DISC Lab
A smaller group joined us for day 2. We started with a few topics of interest chosen by the participants.
We covered the La Magdelena case history (in the inductive source slides). What makes this case history unique is that several EM surveys are employed, each tailored to the exploration step in which it is used. The initial discovery was made with an airborne EM survey, Mise-a-la-Masse to evaluate the continuity of the mineralization. This was followed up by methodological tests where several ground-based EM surveys were tried and assessed for their ability to detect the targets. Finally down-hole EM was applied to find potential targets off-hole from the initial drill holes.
Next, we discussed a HeliSAM example over the Lalor deposit in Canada. In this case history, we highlight a couple take-aways in the approach for the inversion. (1) It was important to “warm start” the inversion by using a starting model that includes a conductive, compact body so that vortex currents can be generated. (2) The early time data were initially thrown away as they were contaminated with power line noise. To assess the impacts of this assumption, an inversion was run that included all of the time-channels. This revealed that by excluding the early time data, erroneous structures were introduced in the near surface.
Tli Kwi Cho is an example of how multiple geophysical data sets, sensitive to distinct physical properties: density, magnetic susceptibility, electrical conductivity and changeability can be inverted and interpreted to obtain a rock model; this was the final case history shown on Day 2. There are three papers that have recently been published by the UBC group: Devriese et al., 2017, Fournier et al., 2017 and Kang et al., 2017 on this work.
Following the presentation of material from us, we turned the stage over to the DISC Lab participants to hear about what they are working on.
Sasha Banaszczyk (slides) is working with MT data over the Capricorn Orogen. She presented MT forward modelling and demonstrated where 1D assumptions break down when representative 2D or 3D structure is introduced, and 1D inversions are no longer adequate.
Perla Pina-Varas is working on inverting MT data collected over the Capricorn Orogen in Western Australia. A significant number of the sites have phases that go out-of-quadrant at low frequencies. There are open questions around what types of conductivity structures can cause this, and she is running inversions to see if such data can be explained by 3D geologic structures.
Mike McWilliams introduced the Deep Earth Imaging Future Science Program which he is leading at CSIRO. It is a forward-looking project aimed at improving subsurface imaging techniques across resource industries in Australia, including energy, minerals and water, and has significant support from the Australian government. As we mentioned earlier, the regolith that covers most of Australia is heterogeneous, often very electrically conductive and has variable thickness across the continent — imaging beneath it presents a significant challenge. Four team leads specializing in: (1) electromagnetics and potential fields, (2) inverse problems, (3) data assimilation and (4) seismic, will lead the teams of postdocs and PhD students on this project.
A few adventures
We had two extra days to take advantage of in Perth. On Saturday, we went for lunch with Mike and his wife, Michelle, and then walked through King’s park and botanical gardens. There is a stunning memorial for the Australians who fought in World War I and II (image on the left). We watched rain clouds roll through which resulted in some pretty dramatic lighting.
On the second day, we went to Yanchep national park which is just outside of Perth. There we saw both kangaroos and koalas, walked through a limestone cave (crystal cave) and attended a talk about Aboriginal culture and practices in Australia.
Thanks to the ASEG and in particular Emma Brand and Kathlene Oliver for helping organize the Perth DISC!